Luminaires for providing general illumination to an area are well known and often used in outdoor lighting applications including roadway and sidewalk lighting, parking lot lighting, and residential area lighting. Luminaires also find use in underground applications, such as in the lighting of tunnels, shafts and platforms.
Currently available luminaires incorporate various light sources including incandescent, fluorescent, halogen, high intensity discharge and light emitting diodes (LEDs). Incandescent light sources are highly inefficient due to significant amounts of energy dissipated as heat. Moreover, although more efficient than incandescent light sources, fluorescent lamps can suffer shorter lifetimes depending on operating environment factors.
Comparatively, LED light sources offer several advantages including high lighting efficiency, long lifetimes, typically between 10,000 and 60,000 hours of operation, resistance to physical or mechanical shock and rapid lighting response time. Long lifetimes are an attractive property of LEDs for underground lighting applications. In underground transportation systems such as subways and train tunnels, for example, replacement of luminaire light sources is costly as replacement procedures often require closing of underground passageways. Closing passageways of underground transportation systems can potentially result in disruption of services and loss of revenue. As a result, long lifetime luminaire light sources are generally desirable.
Nevertheless, LEDs exhibit several disadvantages which challenge their use in luminaire constructions, including luminaires used for underground lighting applications. The performance of an LED, for example, is largely dependent on the temperature of the operating environment. Operating an LED in high ambient temperatures can lead to overheating and device failure. Moreover, LEDs generally project a narrow field of light in comparison with other light sources. As a result, it can be difficult to achieve sufficient illumination over a wide area with LED sources. Furthermore, narrow field light projection can lead to glare capable of interfering with the sight of personnel operating machinery in an underground passageway and can eliminate the stroboscopic effect associated with other light sources/control gear.